The polyolefins industry continues to look for new and better catalyst systems. Ziegler-Natta catalysts are a mainstay, but single-site (metallocene and non-metallocene) catalysts are making inroads. Among other benefits, single-site catalysts can provide polymers with narrow molecular weight distribution, reduced low molecular weight extractables, and enhanced incorporation of α-olefin comonomers. Traditional metallocenes incorporate one or more cyclopentadienyl (Cp) or Cp-like anionic ligands such as indenyl, fluorenyl, or the like, that donate pi-electrons to a central transition metal. In other non-metallocene single-site catalysts, ligands often chelate to the metal through two or more electron donor atoms.
Single-site complexes are normally used in combination with activators, particularly alumoxanes such as methylalumoxane (MAO), triarylboranes (e.g., triphenylborane, “F15”), or ionic borates (e.g., triphenylcarbenium tetrakis(pentafluorophenyl)borate, “F20”). Alumoxanes are less expensive than ionic borates, but they must be used at high aluminum:transition metal mole ratios (typically >1000:1). While the single-site complexes are often expensive to make, the more expensive part of the catalyst system is usually the activator.
Researchers at W.R. Grace & Company recently described catalyst systems that employ, as a combined support-activator, an “agglomerated metal oxide/clay” (see, e.g., U.S. Pat. No. 6,559,090). These support-activators are used in combination with a transition metal complex, such as a conventional metallocene or constrained-geometry complex, to polymerize olefins. As noted in the abstract, the support-activator is a “layered material having a negative charge on its interlaminar surfaces and is sufficiently Lewis acidic to activate the transition metal compound for olefin polymerization.” The examples show the advantages of using the agglomerated metal oxide/clay versus spray-dried clay alone or spray-dried silica alone.
According to the '090 patent, it is preferred to “preactivate” single-site complexes by alkylating them, i.e., by replacing electron-withdrawing ligands such as chloride with “at least one less electronic withdrawing L group (e.g., alkyl) which is more easily displaced . . . by the support-activator to cause activation at the metal center Z” (see column 20). “Preactivation permits one to . . . eliminate the use of expensive methylalumoxane or ionizing agents such as boron containing activators (or co-catalysts).” None of the examples employs an activator other than the support-activator.
Indenoindolyl complexes are a well-known class of organometallic complexes used in single-site olefin polymerization catalysts. For some examples, see U.S. Pat. Nos. 6,232,260, 6,559,251, 6,756,455, and 6,794,468. The complexes have not been tested in combination with agglomerated metal oxide/clay support-activators.
Improving catalyst activity is a continuing battle in the field of single site-catalyzed olefin polymerization. Generally, the less catalyst needed, the lower the process cost and the better the ultimate polymer properties. There is also a need for identifying catalysts and methods capable of providing polymers with relatively high molecular weight. While polyolefin molecular weight can easily be reduced by adding hydrogen or another chain-transfer agent, it is more problematic to find ways to increase molecular weight. New ways to capitalize on the inherent structural flexibility of the indenoindolyl ligand framework are also needed. Finally, there is always a need for catalysts and methods that incorporate comonomers more efficiently. This reduces the amount of comonomer that must be charged, and also reduces the amount of comonomer that needs to be recovered and recycled.